The present invention relates to a method for removing harmful constituents from a gas, and respectively to a method for removing NOx and SOx of low density from air.
Regarding the harmful gas removal apparatus, various oxidation or reduction catalyzers to treat discharged gas from motor vehicles, power plants or the like have been developed. Those harmful gas removal apparatus are constructed to be operable for high density harmful gases of several hundreds ppm and at high temperatures of several hundreds to 1000.degree. C. In design, the removal of low density harmful gases of several ppm from the air is not taken into consideration.
A method of removing trace amounts of harmful gas from air has been developed (e.g., Published Unexamined Japanese Patent Application No. Hei. 1-218622). Also a ventilation system for car road tunnels using harmful gas removal method has been developed (e.g., Published Unexamined Japanese Patent Application No. Hei. 3-233100).
The harmful gas removal method is based on the fact that when a mixture of titanium dioxide (TiO.sub.2) and active carbon or a mixture of the former and iron series metal oxide (Fe.sub.2 O.sub.3, NiO, ZnO, etc.) is illuminated with light, NOx and SOx of low density can efficiently be removed at room temperature. This method is able to remove low density harmful gases of several ppm or less in the air without any additional process, for example, a heating process, at low cost.
In this method, it could be considered that titanium dioxide or iron series metal oxide as the photocatalyzer is activated by light applied thereto. The activated photocatalyzer cooperates with the active carbon to convert the harmful gas to another material, for example, NOx to nitric acid ions (NO.sub.3.sup.-) and SOx to SO.sub.4.sup.2-, and catches the converted material. The mixture of the photocatalyzer and active carbon (referred to simply as photocatalyzer herein after) is gradually deteriorated in its activity since reactive product is deposited thereon. However, after the reactive product is washed away from the photocatalyzer by water, and the photocatalyzer is dried, the photocatalyzer can be used again.
In these harmful gas removal methods the photocatalyzer, as shaped grains, is fixed on the adhesive-coated surface of a fixed member, and thus, the photocatalyzer is brought into contact with harmful gas in a fixed state.
An apparatus for realizing the harmful gas removal method as just mentioned is illustrated in FIG. 5. As shown, a photocatalyzer layer 2 is layered on one side of the inner wall of an upright reactive chamber 1. Light sources 3 for illuminating the photocatalyzer layer 2 are provided on the other side of the inner wall. A series of nozzles 4 and a heater 5 for drying the photocatalyzer layer 2 are disposed along the photocatalyzer layer 2 within the reactive chamber 1. A washing water tank 7 is placed under the reactive chamber 1, with a valve 6 interconnecting them.
The gas subjected to the treatment is fed into the reactive chamber 1 through an inlet 8 formed at the top of the chamber. Within the chamber, the gas comes in contact with the photocatalyzer layer 2 by which the harmful gas is removed from the gaseous body. The gas thus cleaned is discharged from an outlet 9 formed in the lower portion of the chamber. When reactive product is deposited on the photocatalyzer layer 2 and the activity of the layer deteriorates to a certain degree, the feeding of the gas is stopped. The photocatalyzer layer 2 is washed by water spouted out of the nozzles 4, and then dried by the heater 5. The water dropping from the photocatalyzer layer is collected in the tank 7 through the opened valve 6. The thus collected water, if necessary, is processed for harmful contents removal and discharged as waste water.
The harmful gas removal apparatus thus constructed has the following problems.
(1) The regaining process of restoring the deteriorated photocatalyzer layer by the washing and drying operations must be performed within the reactive chamber since the photocatalyzer layer is fixed in the chamber. When the washing device (nozzles 4 in the above example) and the drying device (heater 5) are installed within the reactive chamber, these devices must be laid out so as not to interrupt the air flow and the passage of light from the light sources 3. Thus, proper arrangement of these devise results in complexity of the apparatus. PA0 (2) The fixed side of the photocatalyzer layer in contact with the inner wall of the chamber is not subjected to reaction with the harmful gas. And it is very difficult to uniformly illuminate the wall of the chamber on which the photocatalyzer layer is fastened with light from the light sources. Accordingly, the reaction process is inefficient. PA0 (3) To increase the contact area of the photocatalyzer, the surface area of the fixed member must be increased. The result is apt to increase the size of the harmful gas removal apparatus. It is difficult to increase the contact area of the photocatalyzer while reducing the apparatus size. PA0 (4) The reaction process must be stopped during the photocatalyzer regaining process. PA0 (5) Removal of the deteriorated photocatalyzer and placement of fresh photocatalyzer are not easy.